Field of the Invention
The present invention relates generally to engineered cementitious composite materials, and more particularly to a highly workable polyvinyl alcohol based engineered ductile mortar.
Background Discussion
Mortar based engineered cementitious composite compositions (“ECCs”), frequently referred to as “bendable concretes,” are known. The dramatically increased strain capacity relative to conventional Portland cement has made them highly desirable for use in myriad applications. Unlike conventional cement-based concretes, ECCs do not include aggregate, but substitute for aggregate some kind of fiber material, typically a polymeric fiber of some kind. As an organic material from which reinforcement fibers can be made, polyvinyl alcohol has proven to have superior qualities, principally owing to the enhanced ductility and tensile strength achieved by the composition when set.
The challenge in designing engineered cementitious composites, including PVA-based ECCs, is to provide a composition with rheological properties that make it easy to use—for instance, pumpable and flowable for spray applications, or workable for trowel applications. In consequence, a number of admixtures have been devised to enhance the rheological properties of ECCs. The most creative and prolific intelligence directed to the task has been that of Victor C. Li, Ph.D., a professor in the Department of Civil and Environmental Engineering at the University of Michigan, Ann Arbor, and director of the Advanced Civil Engineering Material Research Laboratories. Dr. Li has devised a number of remarkable ECCs with improved self-compaction, high early strength, and pumpability for spray applications. See, for instance, U.S. Pat. Nos. 6,809,131; 7,241,338; and 7,799,127.
On the other hand, Dr. Li and his colleagues have yet to devise a composition that has properties desirable for commercial use by industry and governments. This is largely because achieving both the desired tensile strength and the ductility necessary to avoid catastrophic failure, the use of some kind of dispersant is needed to ensure that the fiber particles are uniformly (or at least well) dispersed. To create a homogeneous mix, several kinds of “superplasticizers” have been identified and/or devised, nearly all of which comprise carboxylic acid polymers and copolymers. These chemicals act both as dispersants to prevent particle aggregation and bulking, and to improve the rheological properties, principally flow characteristics, of the ECCs. They also reduce the amount of water needed in the composition, and in consequence they are also known as “high range water reducers.” The state of the art in “superplasticizers” for ECCs is currently found in the use of polycarboxylate ether-based superplasticizers (“PCEs”). They can be added to cement in small amounts (e.g., 0.15-0.3% by cement weight), while still providing good fiber particle dispersion and dramatic water reduction.
The shortcoming and problem with the use of PCEs in ECCs, a problem that remains to this day, is that the product is effectively unworkable once applied to a surface. A solution is required, and it is to that end that the present invention has been developed.